Forest fires cause extensive damage when they occur. Both plant and animal life are harmed during these events. The severity of a forest fire depends on how much plant life is available to burn – the more available dry plant material, the more serious the fire and the more rapidly it will spread.

Order of Reaction

In the reaction
, the rate of the reaction is directly proportional to the concentration of
raised to the first power. That is to say,
. A
first-order reaction
is a reaction in which the rate is proportional to the concentration of only one reactant. As a first-order reaction proceeds, the rate of reaction decreases because the concentration of the reactant decreases (
Figure
below
). The graph of concentration versus time is curved. The reaction rate
can be determined graphically by the slope of a tangent to the curve at any point. The rate of the reaction at the time shown with the red triangle is given by:

This graph shows how the concentration of a reactant changes as a reaction proceeds. The rate of the reaction is determined at any point by measuring the slope of a tangent to the curve.

The rates of some reactions depend on the concentrations of more than one reactant. Consider a reaction in which a molecule of
collides with a molecule of
to form product
.

Doubling the concentration of
alone would double the reaction rate. Likewise, doubling the concentration of
alone would also double the rate. The rate law must reflect the rate dependence on both reactants.

This reaction is said to be first order with respect to
and first order with respect to
. Overall, it is a second-order reaction. The rate law and the order of a reaction must be determined experimentally.

Summary

A first-order reaction is described.

Practice

Read the material at the link below and answer the following questions:

What is a zero-order reaction?

What is a second-order reaction?

How is the order of a two-reactant reaction determined?

Review

What is a first-order reaction?

How is the instantaneous rate determined?

How do we determine rate law and reaction order?

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Description

Describes a first-order reaction.

Learning Objectives

Describe the kinetic characteristics of a first-order reaction.

Vocabulary

first-order reaction:
A reaction in which the rate is proportional to the concentration of only one reactant.